The development of water- and oil-repellency is attributed to low surface energy of fluorine atoms, and in generally, fluorine-containing acrylate polymers are widely used as the active ingredient of water- and oil-repellents. Various previous studies have shown that the presence of crystallinity in a fluorine-containing polymer is practically necessary for the development of water- and oil-repellency. Particularly, it has been recognized that the hydrophobicity of a fluorine-containing polymer depends on the number of carbon atoms of a side-chain fluoroalkyl group, and that crystallinity is developed when the number of carbon atoms is 8 or more (see Non-Patent Document 1).
Therefore, it has been considered that fluorine-containing polymers having a short fluoroalkyl group (i.e., a side-chain fluoroalkyl group having 7 or less, particularly 6 or less, carbon atoms) have low or no crystallinity, and thus cannot provide sufficient water-repellency for practical use. Moreover, since water- and oil-repellents are required to have washing durability, friction durability, etc., fluorine-containing acrylate polymers having long side-chains are used.
However, polymers having a side chain fluoroalkyl group containing 8 or more carbon atoms are very hard because of their high crystallinity, and causing the problem that flexibility of textile products is impaired. There are other problems such as discoloring of fibers because the water-repellent treatment necessitates high temperature curing. In addition, since polymers having a long side-chain fluoroalkyl group containing 8 or more carbon atoms are excessively hydrophobic, a large amount of emulsifier is required for the production of water- and oil-repellents.
The present applicant has previously proposed a water- and oil-repellent comprising an aqueous emulsion obtained by dispersing a copolymer of a fluoroalkyl group-containing polymerizable monomer and vinylidene chloride or benzyl (meth)acrylate, and a carboxyl group-containing water soluble polymer in water using a nonionic surfactant. The fluoroalkyl group of the fluoroalkyl group-containing polymerizable monomer to be copolymerized with vinylidene chloride or benzyl (meth)acrylate is specified to be a perfluoroalkyl group having 4 to 20 carbon atoms; however, the perfluoroalkyl group used in the polymerization example is specified to be a mixture of perfluoroalkyl groups having 6 to 14 carbon atoms, 9.0 on average (see Patent Document 1).
Therefore, it is considered that if fluorine-containing polymers having a short side-chain fluoroalkyl group containing 6 or less carbon atoms can develop water- and oil-repellency, water- and oil-repellents that can impart excellent flexibility and washing durability to water- and oil-repellent treated fibers can be obtained.
There has also been proposed a surface-treating agent exhibiting excellent water repellency, oil repellency, and antifouling property obtained by using a polymer having a short fluoroalkyl group containing a side chain of 1 to 6 carbon atoms; however, the fluoroalkyl group-containing monomer used herein is a fluorine-containing monomer compound represented by the general formula:Rf—Y—O—CO—CX═CH2                 X: F, Cl, Br, I, CFX1X2, CN, a fluoroalkyl group having 1 to 20 carbon atoms, a benzyl group, or a phenyl group        Y: an aliphatic group having 1 to 10 carbon atoms, an aromatic or cyclic aliphatic group having 6 to 10 carbon atoms, a CH2CH2NR1SO2 group, or a CH2CH(OY1)CH2 group        Rf: a fluoroalkyl group having 1 to 6 carbon atomsAcrylic acid derivatives in which X is H are shown as comparative production examples, and methacrylic acid derivatives in which X is CH3 are intentionally excluded (see Patent Document 2).        
There has also been proposed a water- and oil-repellent composition containing, as an essential ingredient, a copolymer comprising, as polymerization units, (a) an Rf group-containing monomer that forms a homopolymer having no melting point or a melting point of 55° C. or less attributable to fine crystals derived from the Rf groups of the homopolymer and having a glass transition temperature of 20° C. or higher; and (b) an Rf group-free monomer having a crosslinkable functional group. The water- and oil-repellent composition can impart excellent water- and oil-repellency to products treated at a low temperature, allows water- and oil-repellent treatment resulting in soft touch, and exhibits excellent durability (see Patent Document 3).
In Patent Document 3, a perfluoroalkylalkyl(meth)acrylate is used as the component (a) monomer; however, a specific monomer, such as a 2-butanone oxime adduct, pyrazole adduct, ε-caprolactam adduct of ω-isocyanate alkyl(meth)acrylate, is used as the component (b) monomer in the copolymerization reaction.